Billions of years of exposure to galactic cosmic rays have forged a radiated crust between 15 and 20 meters deep on the surface of the interstellar comet 3I/Atlas. The James Webb Space Telescope identified this altered outer layer during observations conducted in August 2025, when the celestial body was located 445 million kilometers from Earth. As the third confirmed interstellar object in history, the comet carries physical evidence of its long journey across the Milky Way.
Scientists from the Royal Belgian Institute of Space Aeronomy coordinate the research, which documents the chemical conversion of carbon monoxide into carbon dioxide across the damaged surface. Originating from a foreign star system, 3I/Atlas reached its perihelion—the point of closest approach to the Sun—on October 29, 2025, at a distance of 1.36 astronomical units, triggering the sublimation of its exterior shell.
Traveling along a hyperbolic orbit at a velocity exceeding 210,000 kilometers per hour, the object’s trajectory confirms its origin outside the Solar System. Key baseline parameters established by researchers for the visitor include:
- Discovery date: July 1, 2025, detected by the ATLAS survey system in Chile.
- Coma gas profile: Dominated by carbon dioxide at volumes eight times higher than water.
- Estimated age: Up to 7 billion years, making the object older than the Solar System.
Galactic radiation converts carbon monoxide into a deep surface shell
High-energy particles originating outside the Solar System, known as galactic cosmic rays, bombarded the icy surface of the comet over billions of years. This continuous chemical processing transformed carbon monoxide into carbon dioxide, building the 15-to-20-meter superficial crust observed by astronomers.
Computer simulations modeled after comet 67P, which was studied by the European Space Agency’s Rosetta mission, demonstrate that a billion years of radiation exposure is sufficient to generate a shell of this thickness. The absence of a surrounding heliosphere allowed these energetic particles to interact directly and intensely with the surface ice.
Journey outside the heliosphere leaves the exterior unprotected
Unlike local comets, 3I/Atlas traveled through the galaxy without the protective bubble of the Sun’s heliosphere, which filters out deep-space radiation for objects native to the Solar System. This unshielded environment subjected the traveling body to relentless particle collisions that chemically reshaped its exterior.
Comets formed within the Solar System retain their original surface compositions largely intact thanks to this magnetic barrier, setting them apart from the interstellar visitor. On 3I/Atlas, the outer crust accumulated radiation damage on a galactic scale, though the chemical alterations remained confined to the upper layers.
The findings demonstrate how prolonged exposure to deep-space radiation modifies astronomical bodies traveling along interstellar trajectories. Astronomers continue to cross-reference these measurements against observations of local comets to map the differences in surface evolution.
Solar heating at perihelion releases volatile gases from the interior
Following its October 29, 2025 perihelion passage, rising solar heat began releasing trapped gas and dust from the irradiated crust. This sublimation process strips away the altered outer material and exposes internal layers that may hold preserved, unaltered ice.
Researchers compare spectroscopic data gathered before and after the closest solar approach to determine the comet’s original chemical makeup. During this heating phase, the object’s brightness increased rapidly, accompanied by an intense blue hue that signaled the release of volatile gases.
The surrounding coma exhibits a heavy concentration of carbon dioxide, an atypical pattern when compared to standard solar comets. Space-based observatories, including the SOHO and STEREO-A satellites, also recorded active gas jets erupting from the surface while the body was still at cold distances.
Follow: Interstellar comet 3I/ATLAS yields new flight path calculations for deep space origin
NIRSpec instrument measures record carbon dioxide ratios in the coma
Data collected by the Near-Infrared Spectrograph (NIRSpec) aboard the James Webb Space Telescope confirmed elevated concentrations of carbon dioxide within the coma. This high abundance indicates that 3I/Atlas originally formed in cold, carbon-dioxide-rich icy regions near its parent star.
While the radiation-modified crust blankets the exterior, the deeper nucleus preserves primitive materials from the ancient Milky Way. The distinct blue tint observed in the coma results directly from solar heating acting on carbon molecules released into space. Specific measurements from the spectroscopic analysis include:
- Carbon dioxide to water (CO2/H2O) ratio: 7.6 ± 0.3, the highest ever recorded in a comet.
- Core gas output: Emissions of 129 kilograms of carbon dioxide per second from the nucleus.
- Ice particle size: Water ice grains smaller than 1 micrometer suspended in the coma.
- Carbon monoxide to water (CO/H2O) ratio: Measured at 1.65 ± 0.09, confirming processing by cosmic rays.
Hubble and Webb telescopes estimate nucleus size and track outward path
First spotted in July 2025 near the constellation Sagittarius, 3I/Atlas follows an open hyperbolic path that will carry it permanently out of the Solar System following its solar pass. Its velocity of 210,000 kilometers per hour exceeds the Sun’s gravitational escape threshold, verifying its interstellar provenance.
High-resolution imaging from both the Hubble Space Telescope and the James Webb Space Telescope detailed the structure of the coma and tail, placing the nucleus diameter between 440 meters and 5.6 kilometers. Observations conducted in August 2025 also revealed a reddish surface coloration alongside ultraviolet emissions.
Initial cometary activity began in May 2025 when the object was still 6.4 astronomical units from the Sun. As it moves away, the comet remains visible to professional astronomical telescopes, though its overall luminosity is steadily decreasing.
Research teams adjusted standard sublimation models to account for the body’s unusual chemical makeup, which includes metallic elements such as nickel. Direct comparisons with the second known interstellar object, 2I/Borisov, point to shared similarities in gas volatility.
Deep internal layers preserve ancient Milky Way chemistry
Interstellar bodies like 3I/Atlas display carbon dioxide levels that surpass water content in their comas, standing in sharp contrast to comets native to the Solar System. Although the irradiated shell dominates the outer 15 to 20 meters, the deeper strata shield primordial galactic chemistry from alteration.
The August 2025 observational dataset confirms this structural division, isolating the role that deep-space radiation plays in altering external visitors. Ground-based telescopes continue to track the structure of the tail and monitor the rate of brightness decay as the object recedes.
This passage allows planetary scientists to calibrate emission models for astronomical bodies subjected to billions of years of radiation. Ongoing telescope monitoring aims to fill remaining gaps in the study of cometary evolution across different star systems.
